plugin: Present fallback window right after assembled UI

When started from gst-play/launch-1.0 show window right after it was created.
No need to idle dispatch window present when changing state, since we are
already on correct thread when opening resources using start() method.

lib/gst/plugin/gstclappersink.h

@@ -54,8 +54,6 @@ struct _GstClapperSink
   GtkWidget *widget;
   GtkWindow *window;
 
-  gboolean presented_window;
-
   /* Properties */
   gboolean force_aspect_ratio;
   gint par_n, par_d;